This invention relates to a method for manufacturing a small-cross-section wire from a wire having a larger cross-section, in which the wire cross-section is reduced by cold-rolling in a plurality of steps.
In such a known method, the cross-section reducing is obtained exclusively by rolling.
This method has some advantages relative to another known method for reducing a wire cross-section, in which the cross-section reducing is obtained exclusively by wire-drawing in a plurality of steps.
In the first place, feeding a new wire to a rolling unit is very simple. It is only necessary to feed the wire to the first stand and it will then be fed automatically to the following stands. The starting of the unit may thus occur rapidly.
The feeding or the feeding anew of a wire to a wire-drawing unit on the other hand is more intricate. The wire-drawing unit with a plurality of steps has to comprise a relatively high number of dies and as feeding the wire to be drawn to the dies requires a lot of time, there results therefrom a substantial time loss whenever a new wire to be drawn has to be fed to the wire-drawing unit or should the wire break. This is particularly true with wires from aluminum alloy having a relatively large diameter, said wires being rather rigid.
In the second place, the danger of a wire breaking during rolling is much less acute than during wire-drawing.
With wire-drawing, the danger of the wire breaking is quite high, mostly as the number of succeeding reducings is rather high and the total reducing rate is rather substantial. The wire may easily be overstressed, particularly when such wire is made from aluminum alloy.
Indeed, during the reducing by a wire-drawing step, too-strong axial tensile stresses may be generated in the wire, and structural damages may occur in said wire. Now it is impossible to avoid generating axial tensile stresses in the wire, due to the friction, the die angle, and the wire return stress. Such axial stresses generate hollows in the location of small faults in the wire material. Such hollows increase during the succeeding steps and may reach such a size that the actual wire cross-section be severely reduced. The wire may then be more easily overloaded and break in such locations.
Moreover the surface area of the wire is subjected to sliding stresses, which may cause damages to the surface, mostly in combination with faults in the material and/or the friction and/or an out of center position of the wire inside the die.
Rolling causes much less sliding stresses on the wire surface, and the danger of damaging the wire surface is much less severe than during wire-drawing.
In the third place, rolling generates an increase in the temperature which is much less substantial than with wire-drawing.
The energy used for wire-drawing splits-up in 45% homogeneous distortion energy, 10% extreme distortion energy, and 45% friction energy. The homogeneous distortion energy is converted for 90% into heat and the friction energy is completely converted into heat. There results therefrom that each wire-drawing step causes an increase in the wire temperature. As the wire cooling between the various steps is limited, the wire temperature increases from one step to the following step, such increase being all the greater as the wire speed is high. This is particularly true when use is made of a sliding wire-drawing unit. Too-high a heating will cause a break in the lubricant layer surrounding the wire, which might generate surface faults.
With rolling on the other hand, the friction energy is much less substantial; generally no more than 15% of the distortion energy. Consequently, the heating during each rolling step is much lower. Moreover, the generated heat may easily be dissipated in the rolls, in such a way that the cooling action is stronger. There results therefrom that there is substantially no heat accumulating in the wire during the various rolling steps.
As the technical features of the resulting wire, such as the breaking stress, the elongation and the resistivity, are dependent on the wire temperature, the accumulating heating during wire-drawing does limit the wire speed for a particular wire-drawing unit and for particular technical features of the wire.
The technical features of the wire are indeed dependent on the cellular structure which is developed during the treatment. The finer such structure is and the higher will be the breaking stress and the resistivity while the elongation is reduced. A higher temperature causes recovery of the cellular structure. By cooling between two dies, such recovery is controlled.
The lack of such accumulating heating during rolling has the advantage that the technical features of the wire are not dependent on the wire speed and that higher wire speeds may be used.
Such independence of the technical features from the speed is however at the same time a drawback, as it is impossible during rolling to control or change the technical features for a given cross-section reducing by acting on the wire speed.
Besides the above, rolling has further drawbacks relative to wire-drawing.
Indeed, the surface of the wire resulting from rolling is substantially lower in quality than the surface of a wire obtained by wire-drawing.
Moreover, it is not possible to obtain with rolling as accurate a size and a shape of the final wire cross-section than with wire-drawing.
Replacing the rolls from the rolling stands is very time-consuming.
The invention has for object to obviate the above drawbacks and to provide a method for manufacturing wire which allows to obtain wire of good quality and with the required technical features, with an increased throughput relative to the known methods.